[AppBanner] Blink implementation of an event when an install banner is shown.

Source/web/WebLocalFrameImpl.cpp

Index: Source/web/WebLocalFrameImpl.cpp
diff --git a/Source/web/WebLocalFrameImpl.cpp b/Source/web/WebLocalFrameImpl.cpp
index f48c85566e5f6a66c23438876de38b3076970bd2..aecb4234c4e47dca63d7c06c73f7fd23e0b7263d 100644
--- a/Source/web/WebLocalFrameImpl.cpp
+++ b/Source/web/WebLocalFrameImpl.cpp
@@ -149,6 +149,7 @@
 #include "core/rendering/RenderView.h"
 #include "core/timing/DOMWindowPerformance.h"
 #include "core/timing/Performance.h"
+#include "modules/app_banner/AppBannerController.h"
 #include "modules/geolocation/GeolocationController.h"
 #include "modules/notifications/NotificationPermissionClient.h"
 #include "modules/presentation/PresentationController.h"
@@ -1943,6 +1944,14 @@ v8::Handle<v8::Value> WebLocalFrameImpl::executeScriptAndReturnValueForTests(con
     return executeScriptAndReturnValue(source);
 }
 
+void WebLocalFrameImpl::willShowInstallBannerPrompt(const WebString& platform, bool* cancel)
+{
+    if (!RuntimeEnabledFeatures::appBannerEnabled() || !frame())
+        return;
+
+    AppBannerController::willShowInstallBannerPrompt(frame(), platform, cancel);
+}
+
 void WebLocalFrameImpl::willDetachParent()
 {
     // Do not expect string scoping results from any frames that got detached